1. Field of the Invention
The present invention relates to a printing apparatus and printing method and in particular, to controlling of the conveying amount of a printing medium when performing interlace printing.
Furthermore, the invention is applicable in any devices that use printing media such as paper, cloth, leather, non-woven fabric, OHP paper and the like, and even metal. Typical examples of applicable devices are office equipment such as printers, copiers and facsimile machines, and industrial production equipment and so on.
2. Description of the Related Art
The recent popularity of personal computers, word processors, facsimile machines, and so on in offices and at home provides printers of various printing methods as information output devices for those devices. And of them ink Jet printers have the advantages of being easily adapted to color printing, low-level noise when being operated, being capable of printing high quality images on a variety of printing media, and furthermore are easily made compact and so on. This enables ink jet printers to be a suitable information output device for a personal use in offices and at home. And of ink Jet printers, serial scan type ink Jet printing apparatus (hereinafter simply called printing apparatus) that perform printing with a printing head scanning on the printing medium can print high quality images at low cost and have therefore, become widespread throughout the market.
Serial scan type printing apparatuses often adopt a printing method called multi-pass method that can print high quality images. In the multi-pass method, as shown in FIG. 1, a printing head scans (main scan) a certain printing area (A1, A2, . . . ) a plurality of times (three times in the example shown in the FIG.), so that printing for the printing area is complementarily performed through that plurality times of scanning. The number of times which the printing area is scanned leaves multi-pass printing to be called printing performed by multi-passes as in the following: for example, multi-pass printing that performs a three-times main scan over one printing area, as shown in FIG. 1, is called three-pass printing. According to the multi-pass printing, it can reduce or eliminate on the printed images unevenness of density and stripes in the main scanning direction caused by variations in the ejection amount and in the ejecting direction of each ink ejection orifice on the printing head, and white stripes (excessive conveying amount) and black stripes (insufficient conveying amount) resulting from conveying accuracy of conveying printing medium performed between the main scans. It can also reduce or eliminate unevenness of color resulting from ejection orders at the time of bidirectional printing that is caused by arrangement orders of the ejection orifices for each color on the printing head in the main scanning direction. Multi-pass printing can thus improve printed image quality.
In addition to such multi-pass printing, an interlace printing method is used to further improve printed Image quality (Japanese Patent Application Laid-open No. 10-157137). The interlace method forms an array of dots whose intervals are smaller than those of the arranged ejection orifices on the printing head in the sub-scanning direction. More particularly, multi-pass printing conveys is the printing medium by only the amount indicated by
(arrangement direction interval between printing elements (e.g., ejection orifice))×N (N: an integer equal to or greater than zero)
between each two of a plurality times of printing, thereby using printing elements different for each of printing by the plurality of times of scanning to complimentarily perform printing. On the other hand, the interlace printing conveys the printing medium by the conveying amount obtained by adding/subtracting 1/M (M: an integer equal to or greater than two) of the arrangement direction interval between the printing elements to/from the reference conveying amount: (arrangement direction interval between printing elements)×N in the case of using the above conveying amount as reference, that is, an amount indicated by
(arrangement direction interval between printing elements)×N+1/M (N: an integer equal to or greater than zero, and M: an integer equal to or greater than two), or
(arrangement direction interval between printing elements)×N−1/M (N: an integer equal to or greater than zero, and M: an integer equal to or greater than two), and thereby making the interval between scanning lines to which each printing element corresponds by conveying the printing medium less than the arrangement direction interval between printing elements.
According to the above described interlace printing method, gaps between ink dots, particularly in the sub-scanning direction (conveying direction of printing medium) can be filled with ink dots, providing a higher resistance to any shift of the dot position in the sub-scanning direction. More specifically, ink droplets ejected from an ink jet printing head is progressively made smaller with increasing quality of a printed image. On the other hand, the miniaturization and densification of ejection orifices themselves are highly restricted technically and with regard to cost, and thus barely change from those in the conventional manner. For example, it is contemplated that the arrangement direction interval between the ejection orifices be about 42 μm (25. 4 mm/600 pixel), being 1/600 inch to form dots on a printing medium with ink droplets with the diameters of about 32 and 42 μm. When the conveying amount of the printing medium is an Integer-multiple of the arrangement direction interval (42 μm) between the ejection orifices as explained in the multi-pass printing, an image is printed with no gaps when the image is printed with dots aligned in the sub-scanning direction with the diameter of a dot being 42 μm, but on the other hand, when the diameter of the dots is 32 μm, the image is printed with a gap between dots in the sub-scanning direction. If gaps (areas where the image can not be printed) thus exist between dots in the sub-scanning direction, a printed image is sensitively affected when a deviation is made in conveying the printing medium in the sub-scanning direction. That is, the smaller the dots are, the lower the resistance to the deviation in the conveying direction is, which usually makes such a deviation recognizable as unevenness in the image. According to the interlace printing methods it would be able to eliminate possibility of occurrence of such gaps between dots and prevent the unevenness in the Image from occurring due to the positional deviation of dot formation
Meanwhile, some ink jet printers select a printing mode that accompanies a change in the conveying amount for each area of the printing medium to be conveyed. For example, so-called margin-less printing is known, wherein the entire parts of a printing paper are used as printing areas without margins. In this printing, when conveying the printing medium such as printing paper, there are printing areas to which printing is performed while the printing medium is conveyed by only one of upstream paired printing medium conveying rollers or downstream paired printing medium conveying rollers, which are provided at an upstream side of the printing areas and at a downstream side, respectively. These printing areas are parts that usually become margins, and are front end and rear end areas of the printing medium, respectively. To these areas printing is performed.
In these areas, however, a conveying accuracy basically deteriorates, and consequently causes deterioration in printing quality, because the printing medium is conveyed by only one set of paired conveying rollers. To avoid the deterioration, when printing is performed for the front end and rear end areas of the printing medium, a selection Is made for a printing mode wherein the number of printing elements to be used is reduced and in addition, the conveying amount of the printing medium is made smaller, which can improve relative conveying accuracy to the printing elements (Japanese Patent Application Laid Open No. 11-291506). In the margin-less printing, printing is performed for each of a front end area, a normal area and a rear end area which is obtained by dividing the printing area on the printing medium into three areas, as shown in FIG. 2.
FIG. 3 is a flow chart showing a process for the margin-less printing. When print data is received in a step S601, the printing medium is fed (step S602) and it is determined whether print data for the front end area exists (step S603). In the case of the existence of the print data, that is, in the case of margin-less printing, a conveying table for the front end area is used to perform printing in the printing mode for the front end area (step S604). When the printing area is determined not to be the front end area in the step S603, the printing area is determined as to whether to be the normal printing area in a step S605. When it is determined to be the normal printing area, a conveying table for the normal area is used to perform printing (step S606). Printing of the printing mode for the normal area is performed until the area is determined not to be the normal printing area any more (step S605). Then, it is determined whether the printing has been completed in a step S607, and if the printing has not been completed, the printing of the printing mode using the conveying table for the rear end area is effected in a step S608.
One example of the conveying tables to be used in each of the printing modes is shown in FIGS. 4A and 4B. These figures show conveying tables in the case of a multi-pass printing method, and taking an example of a case of this method, explain a selection for a printing mode for each area of a printing medium. The number of the ejection orifices (printing element) of a printing head is 128, 60 printing elements among them are used in the front end and rear end areas, and 120 printing elements are used in the normal area. In addition, 16-pass printing that complementarily completes printing of the printing areas with 16 times of scanning is performed in each area. In the tables shown in figures, the conveying amount for each of the 16 times of scanning of phases 0 to 15 is defined for each area In the tables, the conveying amounts are indicated as how many times as long as the interval between printing elements in their arrangement direction as 1. For example, the conveying amount for a phase 0 in the front end area is designated as 10, indicating that the printing medium is conveyed by 10 times as long as the arrangement direction interval between printing elements. When the arrangement direction interval between printing elements is about 42 μm being 1/600 inch, the conveying amount for the phase 0 in the front end area is 420 μm.
In printing that uses the table, the printing medium is conveyed by using the table from the conveying of the printing medium at the phase 0, and with the conveying amount obtained by sequentially shifting a phase by one whenever scanning is performed. For the front end area, first, the printing medium is conveyed by 10 that is the conveying amount of the phase 0, then, the printing head scans in the main scanning direction to perform printing. Next, the phase is shifted by one and the printing medium is conveyed only for the conveying amount zero of the phase 1 (that is, without being conveyed), and then printing is performed while the printing head scans in the main scanning direction. After this, conveying the printing medium and scanning of the printing head are repeatedly performed while similarly shifting the phase only by one. Furthermore, after the phase 15, a return is made to conveying in the phase 0 to continue to perform printing. Although each printing element of the printing head repeatedly scans the same area when the conveying amount is zero, print data in that case is print data obtained by being divided into respective passes of scanning in accordance with the number of these repeated scanning times.
FIG. 4B shows a case where switching is made from the conveying table for the front end area to the conveying table for the normal area at the phase 6 in the table shown in FIG. 4A. More specifically, the printing medium is conveyed up to the phase 5 according to the conveying table for the front end area and the printing medium is conveyed in the phase 6 with the conveying amount of the phase 6 for the normal area.
FIG. 5 is a diagram showing one example of the relation between printing medium conveying including the transfer from the front end area to the normal area and printing head scanning, and showing a case of three-pass printing to simplify an explanation. The conveying amount is 20 in any of the phases of the front end area, and is 40 in any of the phases of the normal area, different from those of the example shown in FIG. 4A. More specifically, reference numerals 701 to 703 designate scans of the printing head for each of passes, and the total number of printing elements of the printing head is 120 which are expressed as equally divided six blocks. Among six equally divided blocks, blocks indicated by horizontal and vertical lines and meshed represent printing elements to be used at the time of printing. Horizontally arranged printing element blocks. e.g., printing element blocks 701a, 702a and 703a complementarily print the images in the three-pass printing.
In the scans 701 to 703, the conveying table for the front end area is used to perform printing, and in the scans 704 to 707, the conveying table for the normal area is used to perform printing. In the scans 701 to 703, since printing is performed with the conveying amount 20 (in three-pass printing), half of all of the printing elements is used. When transfer is made to the normal area on changing to the scan 704, the conveying amount becomes 40 according to the conveying table for the normal area, and the number of printing elements to be used increases, such as the number of printing elements to be used: 80 in the scan 704, 100 in the scan 705, and 120 in the scan 706 and subsequent scans.
As another example of switching the printing mode that accompanies a change in the conveying amounts, processing referred to as the so-called skipping countermeasure is sometimes carried out. Skipping is a phenomenon that the images are disturbed (become uneven) because the conveying amounts sometimes become larger than expected, when the printing medium sandwiched and conveyed by the upstream paired conveying rollers (for example, conveying roller and pinch roller abutted on the conveying roller) is released from the paired rollers. In the skipping countermeasure to prevent the phenomenon, the conveying amounts are temporarily increased in an area where skipping occurs, and the positions of printing elements to be used are greatly changed at the same time to avoid unevenness caused by the skipping.
FIG. 6 is a diagram explaining processing to avoid the skipping, and showing multi-pass printing with the same number of passes as that of the multi-pass printing in FIG. 5. FIG. 6 shows the scans 801 to 805 corresponding to the rear end area, and shows the conveying amounts in the case of printing including skipping countermeasure processing by those scans and the range of printing elements to be used. After printing is performed by the scan 801 (used printing element is indicated by vertical and horizontal line, and meshed block), the printing medium is conveyed by 20 printing element intervals. Then, printing and conveying are carried In the same manner up to the scan 803, and after printing is performed by the scan 803, the conveying amount is set to (20+60) printing element intervals for the skipping countermeasure processing, and the range of printing elements to be used is changed simultaneously (from the lower half of printing head to the upper half). Thus, in the area where skipping occurs, the conveying amounts are made large in advance to avoid the uncertain conveying amount caused by skipping in this way, and then the range of printing elements to be used is changed in accordance with the certain conveying amount of the printing medium. Thereby, the unevenness in the image caused by the skipping can be prevented from occurring.
When the skipping countermeasure processing is carried out, the range of the used printing elements is greatly moved. In this case, unusual printing sometimes occurs if printing is subsequently performed with the range of the used printing elements as it is. For example, in margin-less printing, the printing is generally performed outside the printing medium on printing to the front end and the rear end of the printing medium. For this reason, an absorber is prepared on a platen to absorb ink ejected beyond the printing medium. In this case, since it is general that the printing is performed with printing elements to be used reduced at printing to the front end and rear end of the printing medium, the ink absorber on the platen is provided mostly in accordance with the position of the printing elements to be used when printing is performed to the front end and rear end of the printing medium. Thus, when the skipping countermeasure processing is carried out, the position of the printing elements to be used does not coincide with that of the ink absorber on the platen in some cases. Because of this, after the skipping countermeasure processing is carried out, the range of the used printing elements is required to be returned again to the range of the printing elements to be used before the skipping countermeasure processing.
In interlace printing method, however, unevenness sometimes occurs in the printed images, in the case that switching is simply made to a conveying table which corresponds to the area shown by the example of multi-pass printing as described above. Similarly in the case of skipping, when the range of the used printing elements is returned to the range of the printing elements to be used before the skipping countermeasure processing, unevenness sometimes occurs in the printed images if the processing is simply carried out.
FIGS. 7A to 7C are diagrams showing conveying tables that are used in printing adopting the interlace method in addition to the multi-pass method. The fact that the conveying amount is 10.5 or 9.5 here shows that the printing medium is conveyed with half of the arrangement direction interval between printing elements shifted in the interlace method. In addition, print data is complemented basically with the conveying amount 10.
FIG. 8A shows a relation between printing medium conveying that includes switching a conveying table and printing head scanning, and how printing is performed on the basis of the relation when printing is performed by the interlace method. As shown in FIG. 8A, the interlace printing performs printing while the conveying amount is increased or decreased by 1/N of the arrangement direction interval between printing elements in the sub-scanning direction (N=2 in example shown in FIG. 8A) when printing is performed based on print data that corresponds to the arrangement direction interval between printing elements in the sub-scanning direction. This makes it possible to carry out interlace printing in which dots are formed with 1/N of the arrangement direction interval between printing elements shifted in the sub-scanning direction (N=2 in example shown in FIG. 8A).
FIG. 8A is a diagram showing, by way of example, a case in which printing is performed to the front end area at the time of the margin-less printing. Further, FIG. 8A shows sixteen times of scanning is performed for a unit area consisting of positions 901 and 902 in the sub-scanning direction and an image to be printed to the unit area is completed with the sixteen times of scanning. The positions 901 and 902 are shifted in the sub-scanning direction from each other by ½ of the arrangement direction interval between printing elements.
In printing of the front end area, when print data is received which corresponds to “nozzles to be used” among nozzles as printing elements shown in FIG. 8A, conveying the printing medium is started from the phase 9 of the conveying table shown in FIG. 7A, the printing medium is conveyed with the conveying amount 0 in the phase 9. Then, printing is performed to a position 901 in the sub-scanning direction shown in FIG. 8A with using the nozzle 1512. Next, the printing medium is conveyed with the conveying amount 0 in each case according to phases 10 and 11, and printing is performed to the position 901 in the sub-scanning direction with using the nozzle 1512 similarly to the previous printing in each case. Then, the printing medium is conveyed with the conveying amount 10.5 according to a phase 12, thereby printing is performed to the position 902 in the sub-scanning direction with using the nozzle 1511. Furthermore, after the printing medium is conveyed with the conveying amount 0 according to a phase 13, printing is performed to the position 902 in the sub-scanning direction with using nozzle 1511. Next, when the printing medium is conveyed with the conveying amount 9.5 by a phase 14, printing is performed to the position 901 in the sub-scanning direction again. However, in this printing, the nozzle 1510 is used. After this, printing is performed similarly to the position 901 or 902 in the sub-scanning direction according to a phase 15, a phase 0, . . . , with changing the nozzle used. Thus, printing for an image of the unit area consisting of positions 901 and 902 is completed with the sixteen times of scanning.
An increase or decrease in shift amount of the printing position in the sub-scanning direction is as follows. A shift amount is necessarily −1/N (N=2 in FIG. 8A) in the conveying the printing medium after the printing medium is conveyed to be shifted by +1/N (N=2 in FIG. 8A), and on the contrary, a shift amount is necessarily +1/N (N=2 in FIG. 8A) after the printing medium is conveyed to be shifted by −1/N (N=2 in FIG. 8A), as shown in the example of the conveying table in FIG. 7A. By repeating the above increase or decrease in such shift amounts, the printing can be complementarily printed with the basic conveying amount 10, and the relation between the positions 901 and 902 in the sub-scanning direction which correspond to their own printing elements can be made constant. A shift amount is also increased or decreased in the normal area and the rear end area in the same manner to perform printing as shown by the conveying table in FIG. 7A.
However, when the interlace printing is carried out, there is sometimes a case where a rule of the increase or decrease in the shift amount is not followed as a result of a switching of conveying tables caused by a change in printing areas. In the case that the rule of the increase or decrease is not followed, unevenness occurs in the printed image.
For example, when a switching is made to the table of the normal area from that of the front end area at the phase 5 as shown in FIG. 7B, the printing medium is conveyed In the normal area with the conveying amount 9.5 (phase 6) after being conveyed in the front end area with the conveying amount 10.5 (phase 4) to follow the rule of the increase or decrease in the shift amount. However, when a switching is made to the conveying table of the normal area from the conveying table of the front end area at a phase 7, as shown in FIG. 7C, the printing medium is not conveyed with −1/N, but with +1/N as it is (being conveyed with the conveying amount 10.5 at phase 8 of the normal phase (N=2 in FIG. 8B)) after being conveyed with +1/N (conveying amount 10.5 in phase 4 of front end area (N=2 in FIG. 8B).
When the printing medium is continuously conveyed with the same shift amount +1/N in this way, an image that is printed before or after switching of this continued table is of an image-printed state as shown in FIG. 8B. That is, printing for the unit area consisting of the positions 901 and 902 in the sub-scanning direction is not performed with the sixteen times of scanning, which causes the printed image to have unevenness. More specifically, in scanning after the conveying according to phases 8 and 9 shown in FIG. 8B, normally, the nozzle 1507 used should correspond to the position 901 in the sub-scanning direction and printing should be performed with using the nozzle 1507. However, actually, the nozzle 1507 is located at a position which is deviated from the position 901 in the sub-scanning direction only by (+½) of the printing element arrangement interval×2, as shown by a reference 903. As a result of this, there is a case that the nozzle to be used does not correspond to the position 901 in the sub-scanning direction and then printing is not performed to the position 901 in the sub-scanning direction after the conveyance by the phases 8 and 9. That is, the sixteen pass printing is not performed to the unit area but a fourteen pass printing is actually performed. This phenomenon causes a complementarity of the image for the unit area to be broken, and then causes unevenness in the image. The occurrence of unevenness is not limited to a transfer from the front end area to the normal area, but unevenness sometimes occurs at a transfer from the normal area to the rear end area in the same manner.
In addition, there is a possibility of the occurrence of a similar phenomenon even at the time of shift of the range of printing elements to be used which is post-processing to processing called the skipping countermeasure processing.